Additives: Due to the strong water absorption and polarity of wood powder, and the non-polarity and hydrophobicity of most thermoplastics, the compatibility between the two is poor, and the interfacial bonding force is minimal. Therefore, it is often necessary to use appropriate additives to modify the surfaces of polymers and wood powder in order to improve the interfacial affinity between wood powder and resin. Moreover, high-volume wood powder has poor dispersion in molten thermoplastics and often exists in some aggregated state, resulting in poor melt flowability and difficulties in extrusion molding. Surface treatment agents need to be added to improve the flowability for extrusion molding. At the same time, various additives are also required to be added to the plastic matrix to improve its processing performance and the performance of the finished product, as well as to enhance the bonding force between wood powder and polymer and the mechanical properties of composite materials. Common additives include the following categories:

Coupling agents can create strong interfacial bonding between the surfaces of plastics and wood powder; they can also reduce the water absorption of wood powder and improve the compatibility and dispersibility of wood powder and plastics, thus significantly enhancing the mechanical properties of composite materials. Commonly used coupling agents include isocyanates, cumene hydroperoxide, aluminates, phthalates, silane coupling agents, maleic anhydride grafted polypropylene (MAN-g-PP), and ethylene acrylate (EAA). The general addition amount of coupling agents is 1wt% to 8wt% of the wood powder addition amount. For example, silane coupling agents can improve the bonding strength between plastics and wood powder, enhance the dispersibility of wood powder, and reduce water absorption, while treating wood powder with alkali can only improve its dispersibility but not its water absorption or bonding with plastics. It should be noted that maleate coupling agents and stearate lubricants can react with each other, leading to a decrease in product quality and output when used together.

Plasticizers For resins with high glass transition temperatures and melt flow viscosities, such as rigid PVC, processing difficulties often arise when compounded with wood powder, and plasticizers are often added to improve their processability. The molecular structure of plasticizers contains both polar and non-polar groups. Under high-temperature shear, they can penetrate into the polymer molecular chains, forming a uniform and stable system through the mutual attraction of polar groups. The insertion of their longer non-polar molecules weakens the mutual attraction of polymer molecules, making processing easier. Common plasticizers used in wood-plastic composites include dibutyl phthalate (DOS). For example, in PVC-wood powder composites, the addition of plasticizer DOP can reduce processing temperature, decrease wood powder decomposition and smoke generation, and improve the tensile strength while increasing the elongation at break as the content of plasticizer DOP increases.

Lubricants: Wood-plastic composites often require the addition of lubricants to improve the flowability of the melt and the surface quality of extruded products. The lubricants used are divided into internal lubricants and external lubricants. The selection of internal lubricants is related to the matrix resin used. It must have good compatibility with the resin at high temperatures and produce a certain degree of plasticization, reducing the intermolecular cohesive energy within the resin and weakening the intermolecular friction, thereby achieving the goal of reducing the resin's melt viscosity and improving melt flowability. External lubricants actually play a role in interfacial lubrication between the resin and wood powder during plastic molding processing. Their main function is to promote the sliding of resin particles. Usually, a lubricant often possesses both internal and external lubrication properties. Lubricants have a certain impact on the service life of molds, barrels, and screws, the production capacity of extruders, energy consumption during production, the surface finish of products, and the low-temperature impact resistance of profiles. Commonly used lubricants include: zinc stearate, ethylene bis-fatty acid amide, polyester wax, stearic acid, lead stearate, polyethylene wax, paraffin wax, oxidized polyethylene wax, etc.

Colorant During the use of wood-plastic composites, soluble substances in wood powder tend to migrate to the surface of the product, causing it to lose its color and eventually turn gray. Different products may also develop black or rust spots under certain usage conditions. Therefore, colorants are also widely used in the production of wood-plastic composites. They enable the products to have a uniform and stable color, and the color fading is slow.

Foaming agent: Wood-plastic composites possess numerous advantages. However, due to the combination of resin and wood powder, their ductility and impact resistance are diminished, rendering the material brittle and nearly twice as dense as traditional wood products, thus limiting its widespread use. After foaming, the wood-plastic composites exhibit a favorable cellular structure, which can blunt crack tips and effectively hinder crack propagation, thereby significantly enhancing the material's impact resistance and ductility, while significantly reducing the density of the product. There are various types of foaming agents, with two commonly used ones being endothermic foaming agents (such as sodium bicarbonate NaHCO3) and exothermic foaming agents (azodicarbonamide AC). Their thermal decomposition behaviors differ, exerting varying effects on the viscoelasticity and foaming morphology of polymer melts. Therefore, it is essential to select an appropriate foaming agent based on the specific requirements of the product.

Ultraviolet stabilizers and others The application of ultraviolet stabilizers has also developed rapidly with the increasing demand for quality and durability of wood-plastic composites. It can prevent the degradation of polymers in the composites or the decline of their mechanical properties. Commonly used ones include hindered amine light stabilizers and ultraviolet absorbers. In addition, in order to maintain good appearance and perfect performance of the composites, antibacterial agents are often added. The selection of antibacterial agents needs to consider various factors such as the type of wood powder, the amount added, the bacteria in the environment where the composites are used, and the moisture content of the product. For example, zinc borate can prevent corrosion but not algae.